Thermionic cathode heater and method of making it



Patented June 9, 1936 UNITED STATES THERMIONIG CATHODE HEATER, AND

' METHOD OF MAKING IT Edward R. Wagenhals, West Orange, N. J., assignorto Radio Corporation of America, a corporation of Delaware No Drawing.Application November 25, 1933, Serial N0. 699,708

,2 Claims. (01. 250--27.5)

My invention relates to heaters for use in vacuum electric devices andmore particularly to an insulated heater element for use in anindirectly heated thermionic cathode for electron discharge devices.

A common type of heater for oxide coated cathodes is a non-inductivereverse wound helical coil of tungsten wire insulated with a coatingconsisting of a mixture of aluminum oxide and about 2% of talc orsilica. Such heaters are in general satisfactory, but some times theinsulation becomes somewhat leaky at high temperatures and allows anappreciable amount of stray and uncontrollable current to fiow betweenthe cathode sleeve and the heater wire. Tubes containing cathode heaterswith leaky insulation are not desirable and may be noisy.

In one method commonly used for making cathode heaters a mixture offinely ground aluminum oxide and about 2% of talc or silica is stirredinto a solution of nitrocellulose to form a syrupy suspension, withwhich a coil of ductile tungsten wire is coated by spraying thesuspension on the coil with an air pressure spray gun. The coating isdried and the coated coil heated from five to ten minutes in a hydrogenfurnace to about 1400 C. to 1600 C. At this temperature the coatinghardens on the tungsten wife, which remains ductile.

It has heretofore seemed necessary, in the commercial manufacture ofsuch cathode heaters, to mix with the aluminum oxide insulation sometalc or silica or similar material which softens at about 1400" C. inorder to obtain a hard firm coating of insulation at a firingtemperature at which the tungstenwire remains ductile. Aluminum oxidefree from talc or silica sinters and forms a coherent layer when firedat temperatures of 1800 C. or higher, but when the heaters were fired inthe usual hydrogen furnace in the usual way to the sintering temperatureof aluminum oxide the tungsten heater wire became very brittle andpractically useless.

One of the objects of myinvention is to provide a cathode heater elementconsisting of a ductile tungsten wire insulated by a coherent sinteredcoating of alumina of the kind usually designated in commerce aschemically pure aluminum oxide.

Another object is to provide a cathode heater of ductile tungsten withrefractory insulation such that even at high temperature the electricalleakage between the heater and cathode is practically negligible.

A further object is to provide a method by which a. ductile tungstenheater wire can be coated with a dense coherent coating of chemicallypure aluminum oxide without impairing the ductility of the wire.

In accordance with my invention the heater element consists of atungsten wire which, although covered with a coherent coating ofsintered chemically pure aluminum oxide, is practically as ductile asthe tungsten heater wires of the heaters insulated with a mixture ofaluminum oxide and talc. The heater element is made by coating a ductiletungsten wire with finely powdered chemically pure aluminum oxidesuspended in a binder, such as a solution of nitrocellulose, and thenfired in an atmosphere of exceedingly wet hydrogen at about 1800 C., atemperature at which the chemically pure aluminum oxide sinters into acoherent layer. I have found that under these conditions the ductiletungsten wire retains its ductility, although it is covered withchemically pure aluminum oxide sintered into a dense adherent coating.Prior to my invention, heaters with ductile tungsten wire insulated withaluminum ox de could be had only with aluminum oxide mixed with talc orsilica, as attempts to insulate ductile tungsten wire by sinteringchemi- 'cally pure aluminum oxide on it resulted in heaters with wiretoo brittle to be useful.

, One procedure by which, in accordance with my invention, I haveobtained commercially useful heater elements involves the use ofaluminum oxide containing about .2% or less of impurities. This oxide,generally made by drying precipitated aluminum hydroxide, can beobtained commercially in the form of a powder which is usually sold aschemically pure aluminum oxide. I find it advantageous to heat thispowdered aluminum oxide to a temperature of about 1500 C. until itshrinks slightly and increases somewhat in density, and then to, grindit in a ball mill into a fine powder, of which the average particle sizeisabout two thousandths of an inch and the maximum particle size is notmore than three times the average size. I find it desirable to ball-millthe oxide in a rubber lined mill with pebbles of pure flint or ofmullite, a commercial form of alumina for about 18 hours. This finepowder of dense chemically pure aluminum oxide is suspended in asolution of nitrocellulose or similar decomposable carbonaceous binder,preferably of about the consistency and viscosity of a thin syrup. Iprefer to make the suspension with the smallest amount of solution thatwill give a smooth even coating of oxide when the suspension is sprayedon the tungsten heater wire. A common and useful form of cathode heateris a ductile tungsten wire, wound into a reverse helix as shown in theUnited States Patent to Fredenburgh, 1,980,675,

November 13, 1934, and assigned to the assignee of my application. Thesprayed coating on such a helix may be from 5 mils. to 10 mils. thick,de-

pending on the degree of insulation required.

In accordance with my invention the tungsten wire helix, coated with asmooth uniform coating of chemically pure aluminum oxide andnitrocellulose binder, is dried, preferably by heating it for a shorttime to about 100 C. to drive off the solvent. The helix with the driedcoating can be handled with ordinary care without injury to the coating.After the coated helix is dried, it is fired in an electric furnace in ahydrogen atmosphere to about 1800 C. Ordinarily a firing of 5 to 10minutes is sufficient. I prefer to fire the coated heater coils byputting them in molybdenum boats which are then passed through thehydrogen furnace in the usual way. In accordance with my invention-thehydrogen atmosphere which surrounds the coated heater during the firingin the furnace is kept very wet, practically as wet as possible whilethe heaters are being fired. The carbon in the binder makes the ductiletungsten wire brittle when fired at 1800 C. in the usual hydrogenatmosphere, but the hydrogen atmosphere saturated with water vaporrenders this carbon innocuous to the ductile tungsten. A convenient Wayof Wetting the hydrogen to the desired extent is'to bubble it throughwater kept at a temperature of about to 0., whereby the hydrogen issaturated with water vapor and wetted much more than when it is bubbledthrough Water at room temperature. I have found that if the heaters ofductile tungsten wire coated with a suspension of chemically purealuminum oxide are fired in this very wet or saturated hydrogenatmosphere at about 1800 C., the ductile tungsten wire of the heaters isnot affected or rendered brittle by the carbon in the binder, andremains ductile, and the particles of chemically pure aluminum oxidesinter together into a dense coherent coating which adheres to the wire.To the naked eye this coating seems smooth and uniform, with a dullfinish. Under the microscope the particles of aluminum oxide appear tobe sintered together into a coherent layer. In sintering the particlesdo not melt and flow together, but soften slightly on the surface andstick together. The sintered aluminum oxide forms on the ductile heaterwire an insulating coating which consists of chemically pure aluminumoxide and'is of very low electrical conductivity.

Although ductile tungsten is the preferred metal for the heater coils,molybdenum or other ductile metal of high melting point may be used. Thepure aluminum oxide may, instead of being mixed with a nitrocellulosebinder and sprayed on the heater wire, be mixed with flour paste andextruded as a rod suitable for use as a core rod for the reverse coiledhelix. The extruded rod fired at 1800 C. or higher in very wet hydrogenis strong and is well adapted for use as a core rod for heater coilswith chemically pure aluminum oxide insulation. A heater made of aductile tungsten wire coil mounted on such a core rod and insulated inaccordance with my invention is sturdy, and has the advantage that boththe core and the coating are of chemically pure aluminum oxide.

The ductile tungsten wire of a heater coil coated with chemically puresintered aluminum oxide in accordance with my invention retains itspliability to such an extent that a 5 mil. tungsten Wire coil of thepitch and diameter of the usual cathode heater coils can be pulled outsubstan tially straight without breaking the wire, although the sinteredcoating may break. Prior to my invention a heater with a tungsten coilof such pliability could not be made with insulation of chemically purealuminum oxide.

While I have described a few specific embodiments of my invention, it isto be understood that various changes and modifications are contemplatedand that the invention is limited in its scope only by the appendedclaims.

What is claimed as new is 1. The method of making an electricallyinsulated heater for electron discharge tubes which comprises coating aductile tungsten heater wire wi h chemically pure aluminum oxide powderof particies of average size of about 2 microns and of maximum size notgreater than 6 microns in suspension in a carbonaceous binder andheating said coated ductile tungsten wire to a temperature ofapproximately 1800 C. in an atmosphere of hydrogen saturated with watervapor until said coating is sintered into a hard dense coherent layeradherent to said wire.

2. The method of making an electrically insulated heater for electrondischarge tubes which comprises spraying a ductile tungsten wire with asuspension in a nitrocellulose binder of aluminum oxide containing lessthan 0.2% of impurities and powdered to particles of maximum size ofabout 6 microns and average size of about 2 microns to form a coating onsaid wire, heating said coated wire in an atmosphere of hydrogensubstantially saturated with water vapor to about 1800 C. to remove thenitrocellulose binder, and maintain said coated wire ductile and tosinter the aluminum oxide on said wire into a hard dense coherentcoating adherent to said wire.

EDWARD R. WAGENHALS.

